In recent years, in a communication system including a base station apparatus (hereinafter may be referred to as “base station”) and a terminal device (hereinafter may be referred to as “terminal”), a machine type communication (MTC) terminal has been attracting attention as a new terminal. As one example of the MTC terminal, included are a smart meter that is an electric power meter provided with a wireless communication function, a vending machine provided with a wireless communication function, and a sensor provided with a wireless communication function. The smart meter provided with a wireless communication function transmits data of measured power consumption and others to a base station, for example. The vending machine provided with a wireless communication function transmits sales information, information on inventory in the vending machine, and others to the base station, for example. As the sensor provided with a wireless communication function, included is a global positioning system (GPS) sensor provided with a wireless communication function, and this GPS sensor transmits information on the current position of a user to the base station.
Because the MTC terminal transmits specific data (information) as in the foregoing, as compared with general terminals (smart phones, for example) that transmit and receive various types of data, the type of data and the amount of data desired as objects of communication are small. That is, in the MTC terminal, there are a relatively large number of cases in which “user data in small size” (hereinafter may be referred to as “small data”) such as measurement data is intermittently transmitted.
When the MTC terminal that is in an idle mode (standby condition) performs transmission of small data, numerous control messages are exchanged with a base station to set up a communication channel with the base station (that is, to move into a connection mode). Because the amount of data in small data is small, if numerous control messages are exchanged when setting up the communication channel for the small data, the overhead of control message increases and it is undesirable in terms of utilization efficiency of communication resources.
Consequently, in the 3rd generation partnership project (3GPP), the following conventional technologies have been studied in order for the MTC terminal in an idle mode to perform the transmission of small data with the exchange of a less number of control messages.
As the first conventional technology, there is one that adds the small data to a message 3 (for example, an RRC connection request) in a random access procedure executed by using a signaling radio bearer (SRB) that is a communication channel for control messages (that is, a communication channel in control plane). The random access procedure using the SRB is executed before a data radio bearer (DRB) that is a communication channel for exclusive use of user data (that is, a communication channel in user plane) is set up.
As the second conventional technology, there is one that keeps holding the DRB that has been set up already without releasing it even when the MTC terminal is in an idle mode.
According to the first and second conventional technologies, because it is not necessary to newly set up a DRB even when small data is generated, at the time of transmitting the small data, the exchange of control messages needed for setting up the DRB can be omitted. Consequently, the amount of control messages at the time of transmitting small data can be reduced. Moreover, due to the reduction in the amount of control messages, the power consumption of the MTC terminal is also reduced.
In the following description, as in the first and second conventional technologies, “communication of transmitting or receiving user data without newly setting up a DRB at the time the user data is generated” is referred to as “short-procedure communication.” The setting up of a DRB is performed in response to a service request transmitted from a terminal to a base station, that is, a setting request for DRB. Consequently, the “short-procedure communication” can also be referred to as “communication of transmitting or receiving user data without a setting request for DRB.”
Furthermore, in the 3GPP, studied has been a technology to utilize “small cell” other than “macro cell.” The “cell” is defined based on “communication area” and “channel frequency” of a single base station. The “communication area” may be a whole of the area in which a radio wave transmitted from the base station reaches (hereinafter may be referred to as “range area”) or may be a divided area that is the divided range area (what is called a sector). The “channel frequency” is one unit of frequency that the base station uses in communication, and is defined based on a center frequency and a bandwidth. The “macro cell” is a cell managed by a base station capable of transmitting at high transmission power, that is, a base station having a large range area (hereinafter may be referred to as “macro-cell base station”). The “small cell” is a cell managed by a base station that transmits at low transmission power, that is, a base station having a range area smaller than that of the macro-cell base station (hereinafter may be referred to as “small-cell base station”). That is, as a cell configuration of communication system, the one in which the cells of different sizes are mixed has been studied. For example, as the cell configuration, there can be a configuration in which a plurality of small cells are included within a macro cell (first cell configuration), or a configuration in which a plurality of small cells are arranged regardless of a macro cell (second cell configuration).
Related-art examples are described, for example, in 3GPP TR23.887 V0.8.0.
A terminal present in a small cell is closer in distance to a base station than a terminal present in a macro cell, and thus it can transmit data at transmission power lower than that of the terminal present in the macro cell. Consequently, a terminal can reduce power consumption by transmitting data to a small-cell base station more than by transmitting the data to a macro-cell base station.
Thus, it is conceivable to reduce the power consumption of an MTC terminal by transmitting small data to a small-cell base station only when the MTC terminal is present in a small cell. Moreover, it is conceivable that the MTC terminal that is present in the small cell transmits the small data to the small-cell base station by using short-procedure communication to further reduce the power consumption of the MTC terminal.
However, to date, there has been no technology that efficiently performs short-procedure communication in a cell configuration in which cells of different sizes are mixed.